Dynamic covalent polymers: Reorganizable polymers with dynamic covalent bonds

A new dynamic covalent bond of Se-N: towards controlled self-assembly and disassembly

A new kind of Se-N dynamic covalent bond has been found that can form between the Se atom of a phenylselenyl halogen species and the N atom of a pyridine derivative, such as polystyrene-b-poly(4-vinylpyridine). This Se-N dynamic covalent bond can be reversibly and rapidly formed or cleaved under acidic or basic conditions, respectively. Furthermore, the bond can be dynamically cleaved by heating or treatment with stronger electron-donating pyridine derivatives. The multiple responses of Se-N bond to external stimuli has enriched the existing family of dynamic covalent bonds. It can be used for controlled and reversible self-assembly and disassembly, which may find potential applications in a number of areas, including self-healing materials and responsive assemblies.

Preorganized hydrogel: self-healing properties of supramolecular hydrogels formed by polymerization of host-guest-monomers that contain cyclodextrins and hydrophobic guest groups

Supramolecular hydrogels formed by a host-guest interaction show self-healing properties. The cube-shaped hydrogels with β-cyclodextrin and adamantane guest molecules mend after being broken. The hydrogels sufficiently heal to form a single gel, and the initial strength is restored. Although contact between a freshly cut and uncut surface does not mend the gels, two freshly cut surfaces selectively mend.

Release of polyphenolic drugs from dynamically bonded layer-by-layer films

Layer-by-layer (LbL) assembled films have been exploited for surface-mediated drug delivery. The drugs loaded in the films were usually released via diffusion or the degradation of one of the film components. Here we demonstrate that drug release can also be achieved by exploiting the dynamic nature of hydrogen-bonded LbL films. The films were fabricated from tannic acid (TA), a model polyphenolic drug, and poly(vinyl pyrrolidone) (PVPON). The driving force for the film buildup is the hydrogen bonding between the two components, which was confirmed by Fourier transform infrared (FTIR) spectra. The film growth is linear, and the growth rate of the film decreases with increasing assembly temperature. Because of the reversible/dynamic nature of hydrogen bonding, when soaked in aqueous solutions, the PVPON/TA films disassemble gradually and thus release TA to the media. The release rate of TA increases with increasing pH and temperature but decreases with increasing ionic strength. Scanning electron microscopy (SEM) studies on the surface morphology of the film during TA release reveal that the film surface becomes smoother and then rougher again because of the dewetting of the film. The released TA can scavenge ABTS(+•) cation radicals, indicating it retains its antioxidant activity, a major biological activity of polyphenols.

Toward self-constructing materials: a systems chemistry approach

To design the next generation of so-called "smart" materials, researchers will need to develop chemical systems that respond, adapt, and multitask. Because many of these features occur in living systems, we expect that such advanced artificial systems will be inspired by nature. In particular, these new materials should ultimately combine three key properties of life: metabolism, mutation, and self-replication. In this Account, we discuss our endeavors toward the design of such advanced functional materials. First, we focus on dynamic molecular libraries. These molecular and supramolecular chemical systems are based on mixtures of reversibly interacting molecules that are coupled within networks of thermodynamic equilibria. We will explain how the superimposition of combinatorial networks at different length scales of structural organization can provide valuable hierarchical dynamics for producing complex functional systems. In particular, our experimental results highlight why these libraries are of interest for the design of responsive materials and how their functional properties can be modulated by various chemical and physical stimuli. Then, we introduce examples in which these dynamic combinatorial systems can be coupled to kinetic feedback loops to produce self-replicating pathways that amplify a selected component from the equilibrated libraries. Finally, we discuss the discovery of highly functional self-replicating supramolecular assemblies that can transfer an electric signal in space and time. We show how these wires can be directly incorporated within an electronic nanocircuit by self-organization and functional feedback loops. Because the network topologies act as complex algorithms to process information, we present these systems in this order to provide context for their potential for extending the current generation of responsive materials. We propose a general description for a potential autonomous (self-constructing) material. Such a system should self-assemble among several possible molecular combinations in response to external information (input) and possibly self-replicate to amplify its structure. Ultimately, its functional response (output) can drive the self-assembly of the system and also serve a mechanism to transfer this initial information. Far from equilibrium, such synergistic processes could give rise to evolving, "information gaining" systems which become increasingly complex because internal self-organization rapidly reduces the potential energy surrounding the system.

Catalytic Control of the Vitrimer Glass Transition

Vitrimers, strong organic glass formers, are covalent networks that are able to change their topology through thermoactivated bond exchange reactions. At high temperatures, vitrimers can flow and behave like viscoelastic liquids. At low temperatures, exchange reactions are very long and vitrimers behave like classical thermosets. The transition from the liquid to the solid is reversible and is, in fact, a glass transition. By changing the content and nature of the catalyst, we can tune the transesterification reaction rate and show that the vitrimer glass transition temperature and the broadness of the transition can be controlled at will in epoxy-based vitrimers. This opens new possibilities in practical applications of thermosets such as healing or convenient processability in a wide temperature range.

Metal-catalyzed transesterification for healing and assembling of thermosets

Catalytic control of bond exchange reactions enables healing of cross-linked polymer materials under a wide range of conditions. The healing capability at high temperatures is demonstrated for epoxy-acid and epoxy-anhydride thermoset networks in the presence of transesterification catalysts. At lower temperatures, the exchange reactions are very sluggish, and the materials have properties of classical epoxy thermosets. Studies of model molecules confirmed that the healing kinetics is controlled by the transesterification reaction rate. The possibility of varying the catalyst concentration brings control and flexibility of welding and assembling of epoxy thermosets that do not exist for thermoplastics.

Changes in Network Structure of Chemical Gels Controlled by Solvent Quality through Photoinduced Radical Reshuffling Reactions of Trithiocarbonate Units

Changes in the structure of networks of chemical gels cross-linked by covalent bonds have been investigated using reshuffling (i.e., degenerative exchange) reactions of the covalent bonds. These reactions can be applied to form functional materials including self-healing polymers, plasticity in cross-linked polymers, and shape-memory polymers. Herein, network structures of chemical gels were changed through radical reshuffling reactions of trithiocarbonate (TTC) units, and swelling degrees or network sizes were controlled by solvent quality. The chemical gels were prepared by RAFT copolymerizations of butyl acrylate and a TTC cross-linker, and the swelling degree was different for polymers prepared by solution or bulk polymerization. The cross-linked polymers were swollen in either good or nonsolvents and then exposed to UV light to trigger the radical reshuffling of the TTC units. The degree of swelling and network size in toluene increased in the presence of good solvents, whereas they decreased in nonsolvents. The repetitive changes in the degree of swelling were accomplished by changing the order of exposure to solvents.

Dynamic Hydrogels with an Environmental Adaptive Self-Healing Ability and Dual Responsive Sol-Gel Transitions

Dynamic polymer hydrogels with an environmental adaptive self-healing ability and dual responsive sol-gel transitions were prepared by combining acylhydrazone and disulfide bonds together in the same system. The hydrogel can automatically repair damage to it under both acidic (pH 3 and 6) and basic (pH 9) conditions through acylhydrazone exchange or disulfide exchange reactions. However, the hydrogel is not self-healable at pH 7 because both bonds are kinetically locked, whereas the hydrogel gains self-healing ability by accelerating acylhydrazone exchange with the help of catalytic aniline. All of the self-healing processes are demonstrated to be effective without an external stimulus at room temperature in air. The hydrogel also displays unique reversible sol-gel transitions in response to both pH (HCl/triethylamine) and redox (DTT/H₂O₂) triggers.

Synthetic molecular machines and polymer/monomer size switches that operate through dynamic and non-dynamic covalent changes

The present chapter is focused on how synthetic molecular machines (e.g. shuttles, switches and molecular motors) and size switches (conversions between polymers and their units, i.e., conversions between relatively large and small molecules) can function through covalent changes. Amongst the interesting examples of devices herein presented are molecular motors and size switches based on dynamic covalent chemistry which is an area of constitutional dynamic chemistry.

Silica-like malleable materials from permanent organic networks

Permanently cross-linked materials have outstanding mechanical properties and solvent resistance, but they cannot be processed and reshaped once synthesized. Non-cross-linked polymers and those with reversible cross-links are processable, but they are soluble. We designed epoxy networks that can rearrange their topology by exchange reactions without depolymerization and showed that they are insoluble and processable. Unlike organic compounds and polymers whose viscosity varies abruptly near the glass transition, these networks show Arrhenius-like gradual viscosity variations like those of vitreous silica. Like silica, the materials can be wrought and welded to make complex objects by local heating without the use of molds. The concept of a glass made by reversible topology freezing in epoxy networks can be readily scaled up for applications and generalized to other chemistries.

Bolaform superamphiphile based on a dynamic covalent bond and its self-assembly in water

We have employed a dynamic covalent bond to fabricate a bolaform superamphiphile, which can be used as building blocks for controlled assembly and disassembly. In alkaline environment, one building block bearing a benzoic aldehyde group can react with the other building block bearing an amino group to form a bolaform superamphiphile. It is found that the bolaform superamphiphiles can self-assemble in water to form micellar aggregates. When the pH is tuned down to slightly acidic values, the benzoic imine bond can be hydrolyzed, leading to the dissociation of the superamphiphile. The micellar aggregates will also disassemble, and the loaded guest molecules are released subsequently. This line of research has enriched the family of bolaform amphiphiles, and the resulting assemblies may find application in the field of controlled and targetable drug-delivery in a biological environment.

Oxime linkage: a robust tool for the design of pH-sensitive polymeric drug carriers

Oxime bonds dispersed in the backbones of the synthetic polymers, while young in the current spectrum of the biomedical application, are rapidly extending into their own niche. In the present work, oxime linkages were confirmed to be a robust tool for the design of pH-sensitive polymeric drug delivery systems. The triblock copolymer (PEG-OPCL-PEG) consisting of hydrophilic poly(ethylene glycol) (PEG) and hydrophobic oxime-tethered polycaprolactone (OPCL) was successfully prepared by aminooxy terminals of OPCL ligating with aldehyde-terminated PEG (PEG-CHO). Owing to its amphiphilic architecture, PEG-OPCL-PEG self-assembled into the micelles in aqueous media, validated by the measurement of critical micelle concentration (CMC). The MTT assay showed that PEG-OPCL-PEG exhibited low cytotoxicity against NIH/3T3 normal cells. Doxorubicin (DOX) as a model drug was encapsulated into the PEG-OPCL-PEG micelles. Drug release study revealed that the DOX release from micelles was significantly accelerated at mildly acid pH of 5.0 compared to physiological pH of 7.4, suggesting the pH-responsive feature of the drug delivery systems with oxime linkages. Flow cytometry and confocal laser scanning microscopy (CLSM) measurements indicated that these DOX-loaded micelles were easily internalized by living cells. MTT assay against HeLa cancer cells showed DOX-loaded PEG-OPCL-PEG micelles had a high anticancer efficacy. All of these results demonstrate that these polymeric micelles self-assembled from oxime-tethered block copolymers are promising carriers for the pH-triggered intracellular delivery of hydrophobic anticancer drugs.